Ignimbrites are composed of bomb- to lapilli-sized pumice fragments, and subordinate lithic fragments, embedded in a matrix of vitric, crystal, and lithic ash. The vitric fragments are glass shards devirved from the pulverization of pumice during eruption. On a microscopic scale, rock fabrics vary significantly due to a variations in flow distortion, compaction, welding, and post-depositional processes such as devitrification and vapor-phase crystallization. This textural heterogeneity is described here, with the help of several photomicrographs derived from the classic paper of Ross and Smith (1961).
Nonwelded ignimbrites are distinct from their welded counterparts in both outcrop appearance and microscopic texture. Glass shards, derived from the fragmentation of the vitric bubble walls of pumice vesicles, are well-preserved. They occur as slender branches having platy to cuspate forms, many of which display triple junctions marking the site of the coalesced bubble walls. In many cases, entire vesicles are well-preserved. Although nonwelded, the glass shards commonly display some degree of compaction, marked by the a slight aligned and/or flattening of the vitric forms.
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| Nonwelded tuff from Sumatra with very slight compaction of glass shards. Note the unusually massive shard center right. | Rattlesnake tuff from central Oregon, displaying slightly flattened shards with unbroken glass bubbles, now in oval outline. |
Compaction and welding is evident in the deformation of glass shards and pumice fragments, as demonstrated by: (1) the collapse of Y-shaped shards and bubble walls, (2) the alignment of elongate crystal and lithic fragments, (3) the folding of shards around lithic and crystal fragments, and (4) the collapse of pumice fragments into glassy lenticular masses called fiamme. The degree of welding can be highly variable, often marked by distinctive color changes reflecting variable oxidation states of iron. Under extreme welding, the welded mass has an obsidian-like appearance, often associated with ghost-like impressions of the flattened shards surrounding the crystal and lithic fragments.
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| Welded tuff from SE Idaho. Note marked compression of the shards, but good retension of the shard structures. | Welded tuff from Valles, N. Mex. displaying well-developed parallel alignment of shards and elongate crystal fragments. | Fine-grained, glassy welded tuff showing extreme compaction and molding against crystal fragments. |
LITHIC- AND CRYSTAL-RICH TUFFS
Pumice fragments are much more common in ignimbrites than are lithic and crystal fragments. Lithic clasts are generally cognate or accidental fragments; i.e., not derived from the erupting magma, but rather from wall-rock within, or below, the edifice of the volcano. Magma-derived crystal fragments are common, particularly in the main body of the pyroclastic flow where they become concentrated by the winnowing out of vitric ash from the flow proper, and into the overlying ash cloud. This process, known as elutriation, effectively concentrates the denser crystal fragments into the main body of the flow, relative to the glass shards. Although crystal fragmentation can be partly attributed to percussive interactions during the eruption, recent data suggests that a more likely scenario involves the internal bursting of individual crystals as they ascend through the magma column. Crystals commonly contain small fluid inclusions, the decompression of which will result in rapid gas expansion, and explosion of the crystals accordingly.
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| A lithic fragment of older welded tuff, displaying marked compaction and distortion of shards, residing in a younger ignimbrite that is poorly welded. | Crystal-rich welded tuff from the 74,000 year-old Toba eruption in Sumatra, displaying compressed glass shards molded around the crystal fragments of quartz, feldspar, and biotite. |
DEVITRIFICATION, AXIOLITIC TEXTURES, AND SPHERULITES
Devitrification is a post-depositional process resulting in the crystallization of microlites along the boundaries of the glass shards or within glassy masses. The mineral compositions produced are mainly cristobalite and alkali feldspar. This process is more common in densely welded ignimbrites, where individual glass shards can often be identified by devitrified crystals radiating from the shard walls toward the inner part of the shard to produce axiolitic texture. This term is derived from the axis of the shard, which is typically outlined by the inward-growing microlites. If welding occurs before devitrification begins, the devitrification process may extend across individual shards boundaries, often obliterating shard structures.
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| The narrow, white margins on these glass shards mark incipient devitrification. The interior of the shards remain glassy. | Highly magnified view (note scale) shows axiolitic texture of feldspar and cristobalite along the walls of a large shard representing the walls of several bubbles. |
Devitrification may occur around scattered nuclei to form spherulites, which are delineated by radiating crystals of acicular cristobalite and feldpar. These spherical aggregates are common features in both rhyolitic lavas and felsic ignimbrites. In the latter, their sub-solidus growth typically results in severe destruction of original tuff structures.
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| Spherulites from a welded tuff at Valles, N. Mex. The concentric banding in the spherulites is due to variations in grain size of the aggregates of cristobalite and feldspar. | The radial aggregates of cristobalite and feldspar are well displayed in this very large spherulite. Note also the growth of secondary minerals generating a plumose structure along the spherulite's outer margin. |
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